Inorganic Chemistry, Vol. 17, No. 7, 1978 1855
' H N M R of UO2(HMPA)?'
R. W. Adams, C. G. Barraclough, R. L. Martin, and G. Winter,.lnorg. Chem., 5, 346 (1966). S. J. Gruber, C. M. Harris, and E. Sinn, J . Chem. Phys., 49,2183 (1968). A. P. Ginsberg, R. L. Martin, and R. C. Sherwood, Chem. Commun,
7203026 and MPS 74-23000. We are also grateful to Professor D. N. Hendrickson and members of his group for assistance in collecting low-temperature magnetic susceptibility data.
856 (1967).
A. P. Ginsberg, R. L. Martin, and R. C. Sherwood, Inorg. Chem., 7, 932 (1968).
Registry No. [($-CSHS)2TiC1]2MnC12*2(OC4H8), 55799-99-0; (sS-C5HS)2TiC12, 1271-19-8; ($'-CSHs)2VC12, 12083-48-6; [ ($C5HS)2TiC1]2ZnC12.2C6H6, 54040-41-4; [(&CsH5)2TiBr]zZnBr2, 54004-69-2; [(q5-CSHs)2TiC1]2BeC12, 54004-68-1; [(CH3CSH4)255800-01-6. TiI2ZnCl4,54064-96-9; [(CH3)2Si(C5H4)2Ti]2ZnC14,
R. Jungst, D. Sekutowski, and G. Stucky, J. Am. Chem. SOC.,96,8108 (1974).
D. G. Sekutowski and G. D. Stucky, Inorg. Chem., 14, 2192 (1975). D. T. Cromer and J. T. Waber, Acta Crystallogr., 18, 104 (1965). R. F. Stewart, E. R. Davidson, and W. T. Simpson, J . Chem. Phys., 42, 3175 (1965).
Supplementary Material Available: Figure 9, showing temperature dependence of magnetic susceptibility and effective moment per Ti for [(q5-CH3CSH4)2TiCl]2ZnC12, Table VII, listing observed and and calculated structure factors for [(~5-CsHS)TiCl]2MnC12~20C4H8, Tables VIII-XII, giving magnetic data for [($-
C5H5)2TiC1]2ZnC12.2CsH6, [(ltS-CSHs)TiBr]2ZnBr2.2c6H6, [($CSH5)2TiCl]2BeC12.2C~H6, [(q5-CH3CSH4)2TiC1]2ZnC12,and [(CH3)2Si(~',q5-CSH4)2TiCl]2ZnC12 (24 pages). Ordering information is given on any current masthead page.
References and Notes (1) (2) (3) (4) (5) (6) (7)
L. V. Interrante, Ed., ACS Symp. Ser., No. 5 (1974). D. J. Hodgson, Prog. Inorg. Chem., 19, 173 (1975). M. D. Glick and R. L. Lintvedt, Prog. Inorg. Chem., 21,233 (1976). B. N. Figgis and G. Robertson, Nature (London), 205, 694 (1965). J. Wucher and H. M. Gijsman, Physica (Utrecht), 20, 361 (1954). J. Wucher and J. D. Wasscher, Physica (Utrecht), 20, 721 (1954). A. Earnshaw, B. N. Figgis, and J. Lewis, J. Chem. SOC.A , 1656 (1966).
D. T. Cromer and D. Liberman, J . Chem. Phys., 53, 1891 (1970). D. G. Sekutowski and G. D. Stucky, J. Am. Chem. Soc., 98,1376 (1976). E. Sinn, Coord. Chem. Rev., 5 , 3 13 (1970). R. D. W. Kemmitt in "Comprehensive Inorganic Chemistry", J. C. Bailar, H. J. Emeleus, R. Nyholm, and A. F. Trotman-Dickenson, Ed., Pergaman Press, London, 1973, pp 790-793, and references therein. C. J. Ballhausen and J. P. Dahl, Acta Chem. Scand., 15, 1333 (1961). C. J. Ballhausen and J. P. Dahl, Acta Chem. Scand., 15, 1333 (1961). N. W. Alcock, J . Chem. SOC.A, 2001 (1967). J. C. Green, M. L. H. Green, and C. K. Prout, J . Chem SOC.,Chem. Commun., 421 (1972). A. Nakamura and S . Ottuka, J. Am Chem. SOC.,95, 5091 (1973). J L. Petersen and L. F. Dahl, J . Am. Chem SOC., 97, 6422 (1975). J. L. Petersen and L. F. Dahl, J . Am. Chem. Soc., 96, 2248 (1974). J. L. Petersen and L. F. Dahl, J . Am. Chem. SOC.,97, 6416 (1975). J. L. Petersen, D. L. Lichtenberger, R. F. Fenske, and L. F. Dahl, J . Am. Chem. Soc., 97, 6433 (1975). R. Jungst, D. Sekutowski,J. Davis, M. Luly, and G. Stucky, Inorg. Chem., 16, 1645 (1977).
R. L. Martin, New Pathways Inorg. Chem., 175 (1968). R. D. Dowsing, J. F. Gibson, M. Goodgame, and P. J Hayward, J . Chem Soc. A , 187 (1969).
Contribution from the Department of Physical and Inorganic Chemistry, University of Adelaide, Adelaide, South Australia 5001, Australia
Proton Magnetic Resonance Study of Ligand Exchange on Tetrakis(hexamethylphosphoramide)dioxouranium(VI) GEOFFREY J. HONAN, STEPHEN F. LINCOLN,* and EVAN H. WILLIAMS Received November 23, 1977
'H NMR studies show the rate law for hexamethylphosphoramide (HMPA) exchange on U02(HMPA)42+to be rate = 4(kl k2[HMPA])[U02(HMPA)42t]in CD2C12diluent, where k1(273 K) = 12.5 1.1 s-', = 14.0 i 3.0 kJ mol-', and AS' = -172 & 11 J K-I mol-' and k2(273 K) = 173 f 15 mol-' dm3 s-l, A P = 22.2 f 3.0 kJ mol-', and AS*= -120 f 9 J K-I mol-'. This two-term rate law contrasts with the first-order rate law observed for the exchange of oxygen donor ligands L in some U02L$+ species, and the origins of the two different rate laws are discussed in terms of dissociative and associative exchange mechanisms.
*
+
Introduction In the solid state the presence of four, five, and six oxygen donor ligand atoms in the equatorial plane of dioxouranium(V1) species is well e~tablished.'-~These ground-state occupancies might indicate the possibilities of similar variations in transition-state equatorial occupancies but the monodentate oxygen donw ligand (L) species U02L52+so far investigated in several noncoordinating solvents (L = dimethylacetamide (DMA),4 dimethyl methylphosphonate (DMMP),S trimethyl phosphate (TMP),6 triethyl phosphate (TEP); tetramethylurea (TMU),' and dimethyl sulfoxide (Me2SO)*)have all exhibited a first-order ligand exchange rate law consistent with a dissociative (D) or dissociative interchange (ID) mechanism and a six-coordinate (including the axial oxo ligands) transition state or reactive intermediate. Consequently, ligand exchange studies on UO2(HMPA)?+ (where H M P A is hexamethylphosphoramide) have been carried out in order to determine the effect of the reduced equatorial plane occupancy of the ground state of this species on the ligand exchange mechanism.
Experimental Section Dioxotetrakis(hexamethylphosphoramide)uranium(VI) perchlorate was prepared under dry nitrogen by refluxing hydrated dioxo-
uranium(V1) perchlorate (G. Frederick Smith) (2.5 g) with triethyl orthoformate (10 g) at 320-330 K for 1 h.9.'0 Dry HMPA (Koch-Light) (3.75 g) was then added at room temperature, and the resultant pale yellow crystals were filtered off, washed with dry ether, and pumped down on a vacuum line for several hours. At all times exposure of the product to light was kept to a minimum to minimize the possibility of photochemically induced redox processes." Anal. Calcd for [U02(HMPA)4](C104)2:C, 24.31; H, 6.12; N, 14.18; P, 10.45; U, 20.07. Found: C, 24.16; H, 6.13; N, 14.54; P, 10.5; U, 19.87. The yield was 96%. Analyses for U as UO;+ were made with an ion-exchangetechnique,I2and C, H, and N analyses were carried out by the Australian Microanalytical Service, Melbourne. No explosion hazard was encountered with [U02(HMPA)4](C104)2,but it should be noted that such perchlorate salts are potentially explosive. Solutions of [U02(HMPA),] (C1O4)2and HMPA in CD2C12diluent (CEA 99.4%) were prepared under dry nitrogen in 5-cm3volumetric flasks. Each solution was transferred to a 5 mm 0.d. NMR tube and was degassed on a vacuum line prior to sealing under vacuum not more than 1 h before commencement of the NMR experiment. The redistilled HMPA and CD2C12were thoroughly dried over Linde 4A molecular sieves prior to use. Proton NMR spectra were run at 90 MHz on a Bruker HX90E spectrometer in PFP mode using a deuterium lock. Depending upon the concentration of the sample, up to ten spectra were computer (Nicolet BNC-12) averaged, and these spectra were then digitized
0020-1669/78/13 17-1855$01 .OO/O 0 1978 American Chemical Society
1856 Inorganic Chemistry, Vol. 17, No. 7, 1978
Honan, Lincoln, and Williams
6.5ms
i '
8.9
37
33
41
45
1d/T 12
Figure 2. Semilogarithmic plots of 7 data for the UO2(HMPA);+ system against 1/T. The data for solutions i-v appear in the order of increasing 7 magnitude and the solid lines are the linear regression best-fit lines.
16 270
22
31
250
47
240
74 EXPT
CALC
Figure 1. Experimental and best-fit calculated 'H NMR line shapes
of a U02(HMPA)42+(0.04452 mol dm-3)-HMPA (0.1896 mol dm-3)-CDzClz(1 3.89 mol dm-3) solution. Experimental temperatures (K) and best-fit T values appear to the left and right of the figure, respectively. The doublet of coordinated HMPA is downfield. Table I. Solution Compositions and Coordination Numbers of UO,,+ in HMPA
[U%-
(HMPA),"]? Soln mol dm-3 i ii iii
iv v
0.1561 0.1314 0.04452 0.01306 0.00511
L,
[ HMPA],b [CD,,CI, mol dm-3 mol dm-
0.6647 0.4870 0.1896 0.06094 0.01907
11.58 13.17 13.89 14.65 15.70
0
0.2
0.4
0.6
0.0
[HMPA] mol dm-3
CNC' 4.00 i: 0.05 4-10? 0.05 3.95 i 0.05 3.95 f 0.05 3.95 f 0.05
a Added as [ UO, (HMPA), ](Cl0.Jz. Added as HMPA. CN = number of HMPA molecules coordinated per UO,*+ion as determined from integration of the doublets of coordinated and free HMPA within the temperature range 180-230 K.
onto paper tape (averaging from 400 to 900 data points per spectrum respectively ranging from fast to slow exchange conditions) prior to line-shapeanalysis through a CDC 6400 computer. The spectrometer temperature control was better than f0.3 K. Results and Discussion The doublet arising from coordinated HMPA ( J I H - u ~ = 10.1 Hz) appears downfield from free H M P A ( J 1 ~ - 3 l p= 9.0 Hz) under slow-exchange 'H N M R conditions as may be seen in Figure 1. Over the temperature range 180-230 K, in which chemical exchange induced line shape modification was negligible, the ratios of the free and coordinated doublet area integrations were consistent with U02(HMPA)42Cbeing the greatly predominant species over the 34.8-fold concentration range investigated (Table I), in agreement with other studies.13 It is assumed that the structure of U02(HMPA)42+in solution is similar to that reported in the solid state' where the four H M P A ligands occupy the equatorial plane about the dioxouranium(V1) axis. At higher temperatures, coalescence phenomena (Figure 1) consistent with ligand exchange were observed and the mean site lifetimes ( 7 ) characterizing HMPA in UO2(HMPA)?+ were derived through a complete lineshape analysis using a computer program which minimizes the
Figure 3. Plots of k,, data sets against [HMPA] where k,, decreases in the sequence 300,290,280,270, and 260 K for a given [HMPA] value. The solid lines are the linear regression best-fit lines to eq 2.
mean of the squares of the residuals between the experimental and calculated line shapes to give the best-fit T values.14 Typical best-fit calculated line shapes and T values for solutions (Table I) are shown in Figure 1. (In this particular case the half-widths a t half-height of the doublet components of free and coordinated H M P A and the chemical shift between the doublet centers in the absence of chemical exchange, which are required input in the fitting procedure, varied respectively from 2.18, 2.50, and 23.95 H z a t 240 K to 1.20, 1.80, and 23.20 H z a t 310 K.) Similar line-shape variations were observed for the other four solutions (Table I) but whereas the temperature ranges over which exchange induced line shape modification occurred were similar, such line-shape modification commenced at a lower temperature as [HMPA] increased from solution v to i. The observed first-order exchange rate law for any one of these solutions is given by eq 1.
4/r = 4k,, = (exchange rate)/[U02(HMPA)p]
(1) The derived T values for the five solutions are plotted semilogarithmically against 1 / T in Figure 2 from which it is seen that there is a systematic dependence of T upon [HMPA]. The logarithm of T varied linearly with 1 / T and each data set was subjected to a linear regression analysis to produce the best-fit lines shown in Figure 2, from which best-fit T values were then interpolated. Some of these interpolated k , (= 1/ T ) values are plotted against [HMPA] in Figure 3 in which it is seen that the overall exchange rate law is given by eq 2. A semilogarithmic plot of k,/T and k,/ T against 1/ T yields the kinetic parameters k1(273 K) = 12.5 f 1.1 (0.1) s-l,
'H N M R of U02(HMPA)4'+
Inorganic Chemistry, Vol. 17, No. 7 , I978 1857
4k,,[U02(HMPA)42'] = 4 ( k l + k2 [ ~ A I ) [ u O 2 ( ~ p A ) 4 2 + 1 (2) = 14.0 f 3 (0.1) kJ mol-', and AS* = -172 f 11 (1) J K-' mol-' and k2(273 K) = 173 f 15 (1) mol-' dm3 s-', AH$ = 22.2 f 3.0 (0.01) kJ mol-', and AS* = -120 f 9 (0.03) J K-' mol-', where the error in parentheses represents one standard deviation for the rate constants and one standard error in the activation parameters as derived from linear regression analyses through eq 3.
k = (KT/h) exp(-AH*/RT) exp(AS*/R)
(3 1
As a consequence of the interpolations involved in the derivation of kl and k2, these errors are unrealistically small, and estimated overall errors appear as the large quoted errors. (The latter errors were calculated by assuming a 10%higher value for each of the best-fit lines in Figure 2 at the highest experimental temperatures and a 10% lower value at each of the lowest experimental temperatures and by drawing new lines through each pair of points from which new k,, values were then interpolated. New kl, kz, AH$, and AS* values were derived from these data. The procedure was then repeated with the 10% higher and lower values reversed at the experimental temperature extremes to yield a second new set of k l , k2, A P , and AS*values. These two sets of kinetic parameters are considered to represent the upper limits of systematic error.) The kl term is consistent with a dissociative (D)15 ligand exchange process proceeding through a reactive intermediate, UO2(HMPA)?+. Other authors have shown that a linear free energy relationship exists between AH$ and AS* for predominantly dissociative ligand exchange processes such that small AH$ values and negative AS*values may arise as observed here.6J6 The kz term is consistent with either a dissociative interchange (ID)l5 ligand exchange mechanism in which the rate-determining step is still primarily bond breaking, but with a synchronous exchange of HMPA between first and second coordination spheres also occurring, or an associative (A) l 5 ligand exchange mechanism proceeding through a transition state of stoichiometry UO2(HMPA)?+. The usual strategem of testing for an A mechanism by noting the variation in k2 as the nature of the incoming ligand is varied is inapplicable in this system as the nature of the coordinated and incoming ligand cannot be independently varied. Nevertheless the observation of ground-state UOZLs2+species in solution"* lends plausibility to the postulated U02(HMPA)52+ transition required by the A mechanism. Hexamethylphosphoramide is the strongest oxygen donor (donor number DN = 38.8) characterized by Gutmann" but only occupies four equatorial ligand sites in ground-state U02(HMPA)42+whereas structurally similar T M P ( D N = 23.0) and TEP (probable DN = 23 as DN = 23.7 for tributyl phosphate), which are of similar bulk to HMPA, produce UO2LS2+ ground-state species which undergo exchange through D or ID mechanisms. Overall it is probable that compensatory extension or contraction of both axial and equatorial bonds serve to maintain a virtually constant total bond order in these ground-state species and the UOz(HMPA)32+or UOZ(HMPA)$+ and U02L42+excited species. While a fine balance between steric interactions and donor power probably determines the ground-state stoichiometry of UOz(HMPA)?+, it is likely that the latter factor provides the major stabilization in U02(HMPA)32+. Conversely, in the case
of TEP (which is sterically very similar to HMPA while being a weaker donor) it is probably the steric interactions which tip the balance to the higher coordination number U 0 2 (TEP)52+ground state and dissociative transition state. Some support for these hypotheses may be adduced from solid-state studies which show the mean U=O and U-OP(N(Me)z)3 distances' to be 1.74 and 2.27 A, respectively, in U 0 2 (HMPA)?' whereas in U 0 2 ( H z 0 ) 5 2 +(DN = 18 for H 2 0 ) the U=O and U-OH2 mean distances2 are 1.7 1 and 2.45 A, respectively, which is consistent with an increase in the uranium(V1) to equatorial ligand bond order as the number of such ligands decreases. Unfortunately, there are no other published structures in which the equatorial plane is occupied only by monodentate oxygen donors, but for a given equatorial occupancy by mono- and bidentate oxygen donors an inverse relationship between the U=O distance and the equatorial ligand distance has been ~ b s e r v e d .Additionally, ~ it appears that for similar U=O distances the equatorial ligand distance is greater when the equatorial occupancy is six than when it is fivee3 The ability of H M P A to stabilize species of reduced coordination number is observed'* in Al(HMPA)43+ which undergoes ligmd exchange through an A mechanism (in contrast to the AlL:' specie^",'^ (L = DMF, Me2S0, TMP, DMMP, dimethyl hydrogen phosphite) which undergo ligand exchange through a D mechanism) and CoBr2(HMPA), and CoC12(HMPA), which exhibit the H M P A exchange rate eq 4 whereas the exchange of L in CoLIXz (where X = C1 or Br and I; and L = 2-picoline or triphenylphosphine) occurs through an A mechanism alone.20 rate = 2(kl
+ k2 [HMPA])[COX,(HMPA)~]
(4)
Acknowledgment. This work was supported by the Australian Research Grants Committee. Registry No. [U02(HMPA)4](C104)2, 59954-64-2; UOz(HMPA)42t, 59954-63-1; HMPA, 680-31-9.
References and Notes L. R. Nassimbeni and A. L. Rodgers, Cryst. Struct. Commun., 5, 301 (1976). N. W. Alcock and S. Esperas, J. Chem. SOC.,Dalton Tram., 893 (1977). N. W. Alcock, J. Chem. SOC.,Dalton Trans., 1616 (1973). R. P. Bowen, S. F. Lincoln, and E. H. Williams, Inorg. Chem., 15, 2126 (1976). J. Crea, S.F. Lincoln, and E. H. Williams, Aust. J . Chem., 29, 2183 (1976). J. Crea, R. Digiusto, S. F. Lincoln, and E. H. Williams, Inorg. Chem., 16, 2825 (1977). G. J. Honan, S. F. Lincoln, and E. H. Williams, submitted for publication in J . Chem. SOC.,Dalton Trans. G. J. Honan, S. F. Lincoln, and E. H. Williams, J. Solution Chem., in press. N. M. Karayannis, C. Owens, L. L. Pytelwski and M. M. Labes, J. Znorg. Nucl. Chem., 31, 2059 (1969). P. W. N. M. van Leeuwen and W. L. Groeneveld. Inore. Nucl. Chem. Lett., 3, 145 (1967). H. D. Burrows and T. J. Kemp, Chem. SOC.Reu., 3, 189 (1974). A. I. Vogel, "Quantitative Inorganic Analysis", 3rd ed,Longmans, Green and Co., London, 1961, p 702. A. Fratiello, G. A. Vidulich, C. Cheng, and V. Kubo, J . Solution Chem., 1. 433 (1972). J:Cr&,'S, F'Lincoln, and R. J. West, Aust. J. Chem., 26, 1227 (1973). C. H. Langford and H. B. Gray in "Ligand Substitution Processes", W. A. Benjamin, New York, N.Y., 1966. (a) H. P. Bennetto and E. F. Caldin, J . Chem. SOC.A, 2198 (1971); (b) E. F. Caldin and H. P. Bennetto, J . Solution Chem., 12, 113 (1972). U. Mayer and V. Gutmann, Slruct. Bonding (Berlin), 12, 113 (1972). J. J. Delpuech, M. R. Khaddar, A. A. Peguy, and P. R. Rubini, J . Am. Chem. SOC.,97, 3373 (1975). W. G. Movius and N. A. Matwiyoff, Inorg. Chem., 6, 847 (1967); J . Am. Chem. SOC.,90, 5452 (1968). S. S. Zumdahl and R. S. Drago, Inorg. Chem., 7, 2162 (1968).
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